1) Field of the Invention
The present invention relates to an automatic frequency control circuit (AFC) suitable for receivers within satellite telecommunication equipment.
2) Description of the Related Art
In recent years, with the development of information-oriented society, the satellite links via a communications satellite have been used as a means of setting telecommunications lines between plural stations.
In Japan, two telecommunications satellites, JC-SAT and Super-Bird, owned by private enterprises, (actually, plus two backup satellites) are now in service.
As well-known, the satellite telecommunications is carried out via a transponder in a satellite (stationary satellite) rotating on a stationary orbit over the equator about 36,000 kilometers. The stationary satellite actually drifts in an eight-figured locus (the locus of the projection point on the earth) at an angle viewed from the earth. Since this locus causes variations in frequency due to the Doppler effect, a certain countermeasure is needed to absorb the frequency variations.
At present, frequency bands each with 500 MHz are allocated for the satellite telecommunications. The frequency bands are split to be supplied respectively to transponders arranged in parallel. Hence, in the satellite telecommunications apparatus, it is required that each transponder can deal with any channels (CH) within a frequency band.
Usually, AFC circuits are used to absorb the frequency variations. In addition, to perform the channel switching operation, a synthesizer-type local oscillator for a frequency conversion is used.
FIG. 15(a) is a block diagram showing frequency bands to which transponders in a satellite link are allocated, respectively. FIG. 15(b) is a block diagram showing the frequency spectrum of channels (CH) arranged for each transponder.
FIG. 15(a) shows frequency bands each with an allocated band (500 MHz) to which transponders are arranged, respectively. As shown in FIG. 15(a), a plural of (about 30 or 32) transponders are respectively used within an allocated band. In order to use the radio waves effectively, the transponders are interleaved or alternately arranged for the vertical polarized wave (V-polarization) and the horizontal polarized wave (H-polarization).
The transponders are generally numbered (as Nos. 1 to 32 in FIG. 15). Usually, the telecommunications service supplier allocates transponders to users, respectively. A specific transponder is allocated when the link is used for a long period of time. In the case of a spot use of the telecommunications link, a transponder in vacant state is used according to the operation environment.
FIG. 15(b) shows the frequency spectrum of channels (CH) arranged for each transponder. The channel arrangement has its different form according to a kind of a transmitted signal or of the use aspect. In this case, small capacity, multichannels for telephones or facsimiles are shown. FIG. 15(b) shows a pilot signal or a carrier arranged at the center frequency and a guard band being a space arranged on both sides of the pilot signal. The guard band is arranged to process easily the AFC (to be described later).
In FIG. 15, signals on the telecommunications link are arranged outside of the guard band. The signals are allocated so as to have a constant channel spacing. Each channel is used, if necessary, temporarily. Hence, vacant channels have no signals. For convenience, the pilot signal is arranged at the center frequency of the transponder, but may be arranged at a different frequency.
FIG. 16 is a block diagram showing the theory of the AFC circuit. Referring to FIG. 16, the frequency converter 1 converts an input signal into a signal of a desired frequency. The local oscillator 2 supplies a local signal for frequency conversion to the frequency converter 1. The bandpass filter BPF 4 passes selectively a signal of a desired frequency among signals converted by the frequency converter 1. The reference oscillator (REF) 5 produces a reference signal to synchronize an input signal to a fixed frequency. The phase comparator 6 compares the output from the reference oscillator 5 with the output from the bandpass filter 4. The loop filter 7 smooths the output from the phase comparator 6 to output the outcome to the local oscillator.
In the AFC circuit configuration, an input signal is subjected to a frequency conversion by the frequency converter 1. In this case, a desired transponder is selected among the transponders within an allocated band of 500 MHz by suitably varying the frequency of the local oscillator 2. Then the bandpass filter 4 can take only the pilot signal out of the selected signal.
The phase comparator 6 compares the pilot signal with the reference signal from the reference oscillator 5 and outputs an error signal .DELTA..delta., or a difference between the pilot signal and the reference signal. The error signal .DELTA..delta. is outputted to the local oscillator 2 via the loop filter 7. The local oscillator 2 varies its output frequency according to the error signal .DELTA..delta..
The configuration described above forms a phase synchronization loop to the pilot signal. If the frequency of the reference oscillator 5 is set to a desired pilot frequency fPL, the local oscillator 2 is controlled so as to set the frequency at a fixed value after the frequency converting operation, or the oscillation frequency of the reference oscillator 5, without depending on the variations in frequency of the input signal.
FIG. 17 is a block diagram showing a configuration of a conventional automatic frequency control circuit. In FIG. 17, like numerals are attached to elements similar to those shown in FIG. 16.
The automatic frequency control circuit shown in FIG. 17 is fundamentally identical to that shown in FIG. 16. As shown in FIG. 17, both the voltage controlled oscillator 3 and the frequency divider 10 are added to the automatic frequency control circuit.
The voltage controlled oscillator (VCO) 3 receives an error signal .DELTA..delta. from the phase comparator via the loop filter 7 and then supplies a reference signal corresponding to the error signal .DELTA..delta. to the local oscillator 2. The frequency divider 10 having a frequency division number M is arranged between the local oscillator 2 and the voltage controlled oscillator 3. The local oscillator 2 is constituted as a synthesizer (to be described later).
For the brief explanation, specific values are given in FIG. 17. In this case, the transponder No. 17 of the JC-SAT is used as an example. The transponder No. 17 has the center frequency of an allocated band of 500 KHz.
The center frequency of the transponder No. 17 is 12.50825 GHz. However, since the local oscillation is usually at 11.3 GHz for a video reception, the center frequency is block-converted to 950 to 1450 MHz in the L-band. In accordance with such a situation, the center frequency fRF of the input signal in FIG. 17 is 1208.25 MHz. When the frequency converter I converts the center frequency fIF of the intermediate frequency (IF) band to 140 MHz, the oscillation frequency fLO of the local oscillator 2 is 1068.25 MHz, according to the following formula: EQU fLO=fRF-fIF (1)
Since the channel arrangement in the transponder is set as shown in FIG. 15(b), the frequency fPL of the pilot signal is the same as the center frequency of the transponder, thus expressed as the following formula: EQU fPL=fIF (2)
Therefore, fPL is 140 MHz.
In this case, since 10 or 5 MHz is widely used for a higher stable reference oscillator, the output frequency fREF of the voltage controlled oscillator 3 is selected to 10 MHz.
In order to obtain a frequency fLO of 1068.25 MHz from a frequency fREF of 10 MHz, the local oscillator 2 is formed as a synthesizer type (phase locked loop (PLL)) oscillator including the voltage controlled oscillator 21, the loop filter 22, the phase comparator (PD: phase detector) 23, and the frequency divider 24. The frequency divider 24 divides a signal of an oscillation frequency fLO from the voltage controlled oscillator 21 to 1/N. The phase comparator 23 compares the outcome from the frequency divider 24 with the signal from the frequency divider 10 to produce an error signal. The error signal is inputted to the voltage controlled oscillator 21 via the loop filter 22 to adjust the output frequency fLO of the voltage controlled oscillator 21 in accordance with the error signal.
As shown in FIG. 16, the voltage controlled oscillator 2 may be directly controlled with the output from the loop filter 7. However, the voltage controlled oscillator 21 has a low frequency stability because of its high oscillation frequency and its wide band coverage of 500 MHz. This low stability does not cause any troubles at the AFC locking operation but may cause the operational failure due to a frequency shift at the rising or re-drawing time. This type of configuration is not used in most commercially-used apparatuses. Hence, the synthesizer-type local oscillator 2, as shown in FIG. 17, is widely used.
As described above, if fLO=1068.25 MHz and fREF=10 MHz, the relation is expressed as follows: EQU fLO=N.fR (3)
where fR is an output frequency of the frequency divider 10, or a comparison frequency of the phase comparator 23, and N is a positive integer division number of the frequency divider 24. In this case, since fR is 250 KHz, N=4273 and M=40.
In the AFC configuration shown in FIG. 17, the phase noise of the local oscillator 2 that affects seriously the quality of the telecommunications line will be studied below.
If the PLL oscillator shown in FIG. 17 is used as the local oscillator 2, the output phase noise depends on the noise floor, VCO noise, or the like. Particularly, the noise floor is dominant within the loop band. The noise level is as follows: EQU PNout=PNPD+20.logN (dBc/Hz) (4)
where PNout is a phase noise of the local oscillator 22, PNPD is a noise floor of the phase comparator 23, and N is a frequency division number of the frequency divider 24. There are variations in the noise level of the phase comparator 23 published in many papers. Here, let us use an average PNPD=-140 dBc/Hz. Using the above condition, the noise level PNout, shown in FIG. 17, is calculated as follows: ##EQU1##
The final noise level at the frequency fIF in the output signal is an addition of the noise level at the center frequency fRF in the input signal and the noise level at the frequency fLO in the signal from the local oscillator (voltage controlled oscillator 21) 2. Hence, in order to improve the noise level, it is efficient to reduce the frequency division number N of the frequency divider 24.
Now, let us study the concrete measure that reduces the frequency division number N of the frequency divider 24. It is required that the local oscillator 2 can be varied to a suitable frequency over the 500 MHz band to select a specific transponder. In the case of the JC-SAT, the transponders are arranged at 30 MHz intervals or 15 MHz intervals, in consideration of the vertically polarized wave in an interleaved arrangement.
Hence, the output frequency step of the local oscillator 2 is preferably 15 MHz. Based on the theory of the PLL oscillation, the phase comparator 23 has a comparison frequency fR of 15 MHz. Increasing the frequency fR can decrease the frequency division number N of the frequency divider 24.
The frequency division number N can be decreased according to the mutual frequency intervals. However, since the requirement to the absolute frequency of 1068.25 MHz fixes the comparison frequency fR to 250 KHz, the frequency division number N cannot be reduced. Hence, there is a disadvantage in that the frequency division number N larger than the functionally required value leads to degradation due to noises.